Conventionally, as compressors for compressing refrigerant or air, there have been known single-screw compressors including one screw rotor, a casing for accommodating the screw rotor, and two gate rotors (refer to Japanese Patent Publication No. H02-248678 A).
In the screw compressors, compression chambers are formed by meshing gates of the gate rotors with helical grooves of the screw rotor, and refrigerant is compressed by rotation of the screw rotor and the gate rotors. In this context, oil is injected into the compression chambers for the purpose of lubricating the helical grooves and the gates and enhancing sealability of gaps between the helical grooves and the gates.
In addition, there have been known other screw compressors in which liquid refrigerant, other than oil, is injected into compression chambers or in which intermediate-pressure refrigerant is injected into compression chambers.